[Source: NOAA/ESRL/PSD, http://www.esrl.noaa.gov/psd/ ]
The Tropical North Atlantic Index is Anomaly of the average of the
monthly SST from 5.5N-23.5N and 15W-57.5W. GISST and NOAA OI 1x1 datasets
are used to create the index. Climatology is 1951-2000.